The present invention relates to a process for the production of saturated, high-molecular weight polyesters by condensation of terephtahalic acid, which can be up to 30 molar percent substituted by other dicarboxylic acids, with 1,4-butanediol, which can be up to 30 molar percent substituted by other diols, in two stages at an elevated temperature and in the presence of condensation catalysts.
The production of saturated, high-molecular weight polyesters is ordinarily conducted in a two-stage process. In the first stage, generally called the esterification and/or interesterification reaction, the low-molecular weight polyester building blocks are produced. In the second or polycondensation stage, the low-molecular weight initial products are reacted to high-molecular weight products while the diol is removed by distillation.
For the polycondensation stage during the production of poly(butylene terephthalate), a temperature range of 250.degree.-280.degree. C. has been reported under a pressure of 0.7 mbar in J. Pol. Sci., 4,A-1: 1831 et seq. (1966). The reaction time is about one hour. This reference states that, under the disclosed reaction conditions, a great temperature sensitivity is displayed by the poly(butylene terephthalate). On the other hand, it is necessary to conduct the reaction under a very minor pressure to avoid an even greater thermal load on the polycondensate.
If the temperature load is of too long a duration, degradation reactions occur so that a certain molecular weight cannot be exceeded. However, higher molecular weights are required by practical applications technology, e.g., so that processing by means of an extruder can be successfully conducted. For this purpose, an additional solid-phase post polycondensation follows the polycondensation stage under practical conditions.
Such a mode of operation is expensive in various aspects. Therefore, it would be highly desirable to reduce the number of operating steps. This possibility could be achieved if the poly(butylene terephthalate) could be polycondensed within shorter periods of time. Furthermore, it would be highly advantageous if the polycondensation stage could be accomplished under higher pressures of about 25 mbar, i.e. under a water-jet aspirator. Pressures of below 25 mbar, especially below 1 mbar, require very great expenditures for an operation on a large technical scale.
It is an object of this invention to develop a process for the production of poly(butylene terephthalate) making it possible to obtain the polyester, without a solidphase polycondensation, at sufficiently high molecular weights for extrusion processing. Another object is to effect the polycondensation stage under only a water-jet vacuum.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.